This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-301303, filed Sep. 29, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a battery pack and a portable electronic appliance, particularly, to a battery pack with an improved cycle life and a portable electronic appliance having the battery pack used therein.
2. Description of the Related Art
In recent years, the miniaturization and the reduction of weight of a portable electronic appliance has been drastically promoted in accordance with the technical innovation achieved in the field of the electronics. In this connection, an improvement in the energy density is required in respect of the battery providing the driving power source of the electronic appliance. A nonaqueous electrolyte secondary battery such as a lithium ion secondary battery has a high energy density and, thus, is widely used nowadays as a power source of many portable appliances.
The nonaqueous electrolyte secondary battery developed in the initial stage was mainly a cylindrical nonaqueous electrolyte secondary battery having an electrode group and a nonaqueous electrolyte housed in a cylindrical container having a circular cross section and made of a metal, said electrode group being formed by spirally winging a laminate structure comprising a sheet-like positive electrode, a sheet-like negative electrode and a sheet-like separator interposed between the positive electrode and the negative electrode. Then, the cylindrical container having a circular cross section was changed into a cylindrical container having a rectangular cross section so as to provide a nonaqueous electrolyte secondary battery having a rectangular cross section, which was superior in the volume efficiency to the cylindrical secondary battery having a circular cross section. Further, in recent years, a nonaqueous electrolyte secondary battery, which permits further decreasing the thickness and the weight, has been put to a practical use by using a case consisting essentially of a film material such as a laminate film.
The thickness and the weight of a portable electronic appliance can be decreased by using a nonaqueous electrolyte secondary battery comprising a case consisting essentially of a film material as a power source of the portable electronic appliance. Also, the nonaqueous electrolyte secondary battery of this type exhibits a good heat dissipation efficiency and is advantageous over the conventional cylindrical nonaqueous electrolyte secondary battery in safety. Stated reversely, however, the nonaqueous electrolyte secondary battery of this type is likely to be affected by the ambient temperature and is poor in the cycle characteristics at about 45 to 50xc2x0 C., giving rise to the problem described below when mounted to a portable electronic appliance.
Specifically, parts that tend to generate heat such as a CPU of an electronic computer, a back light of a liquid crystal display device and a booster circuit are incorporated in the portable electronic appliance. As a result, if a battery assembly comprising a plurality of batteries of the nonaqueous electrolyte secondary battery are mounted to such a portable electronic appliance, only one of the secondary batteries, which is positioned adjacent to the heat generating part, is selectively heated so as to generate a large difference in temperature among the batteries of the battery assembly. It follows that a difference in voltage among the batteries of the battery assembly is increased, with the result that a heavy load is applied to one of the batteries of the battery assembly so as to markedly lower the battery characteristics, particularly, the cycle life. What should also be noted is that, when a single battery of a large capacity is mounted to the portable electronic appliance, a part of the battery is heated by the heat generating part, giving rise to inclination in the temperature distribution within the battery. In this case, a charge and discharge reaction is likely to take place nonuniformly so as to markedly lower the cycle life of the battery.
An object of the present invention is to provide a battery pack with an improved cycle life and a portable electronic appliance having the battery pack used therein.
Another object of the present invention is to provide a battery pack capable of suppressing the diffusion of a leaking electrolyte to the outside and a portable electronic appliance having the battery pack used therein.
According to a first aspect of the present invention, there is provided a battery pack, comprising:
a secondary battery unit including at least one secondary battery, the at least one secondary battery comprising an electrode group and an insulating case having the electrode group housed therein; and
a heat conductive sheet provided at at least a part of the surface of the secondary battery unit and having a heat conductivity higher than a heat conductivity of the case.
According to a second aspect of the present invention, there is provided a portable electronic appliance using a secondary battery unit including at least one secondary battery, the at least one secondary battery comprising an electrode group and an insulating case having the electrode group housed therein, comprising:
a battery housing section in which a secondary battery unit is arranged; and
a heat conductive sheet arranged within the battery housing section and having a heat conductivity higher than a heat conductivity of the case.
According to a third aspect of the present invention, there is provided a portable electronic appliance, comprising:
a secondary battery unit including at least one secondary battery, the at least one secondary battery comprising an electrode group and an insulating case having the electrode group housed therein; and
a heat conductive sheet provided at at least a part of the surface of the secondary battery unit and having a heat conductivity higher than a heat conductivity of the case.
According to a fourth aspect of the present invention, there is provided a battery pack, comprising:
a heat conductive sheet;
a thermoplastic film arranged on the heat conductive sheet;
an electrode group arranged on the thermoplastic film; and
a film material thermally fused to the thermoplastic film in a manner to cover the electrode group so as to seal the electrode group.
According to a fifth aspect of the present invention, there is provided a portable electronic appliance, comprising:
a heat conductive sheet;
a thermoplastic film arranged on the heat conductive sheet;
an electrode group arranged on the thermoplastic film; and
a film material thermally fused to the thermoplastic film in a manner to cover the electrode group so as to seal the electrode group.
According to a sixth aspect of the present invention, there is provided a battery pack, comprising:
a secondary battery unit including at least one secondary battery comprising an electrolyte; and
an electrolyte absorbing sheet arranged on at least a part of the surface of the secondary battery unit.
According to a seventh aspect of the present invention, there is provided a portable electronic appliance using a secondary battery unit including at least one secondary battery comprising an electrolyte, comprising:
a battery housing section in which a secondary battery unit is arranged; and
an electrolyte absorbing sheet arranged within the battery housing section.
Further, according to an eighth aspect of the present invention, there is provided a portable electronic appliance, comprising:
a secondary battery unit including at least one secondary battery comprising an electrolyte; and
an electrolyte absorbing sheet arranged on at least a part of the surface of the secondary battery unit.